1. Field of the Invention
The present invention relates generally to aluminum alloy compositions and, more particularly, to improving the surface quality of aluminum ingots produced therefrom through closely controlled alloying additions, which improve downstream processing and yields.
2. Description of the Prior Art
It is well known in the aluminum casting art that various surface imperfections such as pits, vertical folds, oxide patches and the like, which form during ingot casting, can develop into cracks during casting or in later processing. A crack in an ingot or slab propagates during subsequent rolling, for example, leading to expensive remedial rework or outright scrapping of the cracked material. Most ingots are worked in some manner; however, working will not heal a cracked ingot. Surface imperfections in aluminum cast ingots remains a problem in the alloy art.
Working refers to various operations well-known in the metallurgy art, which include hot rolling, cold rolling, extruding, forging, drawing, ironing, heat treating, aging, forming, and stretching, to name a few. In working or forming an alloy, energy is put into the workpiece, but it is not always homogeneously distributed.
The casting of alloys may be promoted by any number of methods known to those skilled in the art, such as direct chill casting (DC), electromagnetic casting (EMC), horizontal direct chill casting (HDC), hot top casting, continuous casting, semi-continuous casting, die casting, roll casting and sand casting. Each of these casting methods has a set of its own inherent problems, but with each technique, surface imperfections can still be an issue. One mechanical means of removing surface imperfections from an aluminum alloy ingot is scalping. Scalping involves the machining off a surface layer along the sides of an ingot after it has solidified.
Aluminum alloys may comprise any of the Aluminum Association (“AA”) registered alloys such as the 1xxx, 2xxx, 3xxx, 4xxx, 5xxx, 6xxx, 7xxx and 8xxx series alloys. Certain alloys, such as 7050 and other 7xxx alloys as well as 5182 and 5083 are especially prone to surface defects and cracking. In the past, beryllium has been added, usually at part per million (ppm) levels to some of these alloys to control surface defects. However, beryllium has been banned from aluminum products used for food and beverage packaging. Further, there have been increased concerns over the health risks associated with factory workers using beryllium and products containing beryllium. For this reason, although beryllium is effective at controlling surface defects in aluminum cast ingots, a suitable replacement is needed.
U.S. Pat. No. 5,469,911 to Parker discloses a method for improving the surface quality of electromagnetically cast aluminum alloy ingots, which includes the addition of 0.01 to 0.04 wt. % calcium prior to the ingot head of an ingot mold. These levels of calcium are significantly higher than the ppm levels employed with beryllium. Such high levels of calcium can adversely affect the properties of the alloy.
U.S. Pat. No. 4,377,425 to Otani et al. discloses using calcium in high iron containing direct chill cast aluminum alloy ingots to minimize the occurrence of dendritic or so-called “fir tree” crystal structures with a grain size of less than 150 microns. This method was particularly useful for AA1000 and AA5000 series aluminum alloys. The effect, if any, of calcium on the surface quality of the resulting ingots was not disclosed by Otani et al.
Historically, in the melting and casting of aluminum alloys, calcium, as well as sodium, were considered to be unwanted elements because of edge cracking problems. These elements typically have been removed from the melt by way of chlorine gas fluxing prior to ingot casting.
There remains a need for an effective alternative to beryllium to prevent surface imperfections such as vertical folds, pits, oxide patches and the like from forming during aluminum ingot casting. Such a method would be instrumental in preventing cracks, which can form during casting or can develop in later processing. Finally, the method preferably would have no adverse affect on alloy properties.